Returns the company's preferred supplier provider together with its favorite branch. While the provider is connected, that branch is the Zuper vendor used to price and pre-fill supplier materials on proposals. data is null (with the message No preferred supplier set) when no provider is preferred, and favorite_supplier_vendor is null when the provider has no active favorite branch. The preferred provider is returned even after it loses its connection, so check connection_status before relying on it. Credentials are never returned. Requires supplier integration to be enabled for the company; otherwise the request fails with 400.